Various improvements in replication (#11949)

- collect real time replication metrics for prometheus.
- add pending_count, failed_count metric for total pending/failed replication operations.

- add API to get replication metrics

- add MRF worker to handle spill-over replication operations

- multiple issues found with replication
- fixes an issue when client sends a bucket
 name with `/` at the end from SetRemoteTarget
 API call make sure to trim the bucket name to 
 avoid any extra `/`.

- hold write locks in GetObjectNInfo during replication
  to ensure that object version stack is not overwritten
  while reading the content.

- add additional protection during WriteMetadata() to
  ensure that we always write a valid FileInfo{} and avoid
  ever writing empty FileInfo{} to the lowest layers.

Co-authored-by: Poorna Krishnamoorthy <poorna@minio.io>
Co-authored-by: Harshavardhana <harsha@minio.io>

cmd/bucket-quota.go

@@ -88,7 +88,7 @@ func (sys *BucketQuotaSys) check(ctx context.Context, bucket string, size int64)
 			return err
 		}
 
-		dui := v.(DataUsageInfo)
+		dui := v.(madmin.DataUsageInfo)
 
 		bui, ok := dui.BucketsUsage[bucket]
 		if !ok {
@@ -115,7 +115,7 @@ func enforceBucketQuota(ctx context.Context, bucket string, size int64) error {
 
 // enforceFIFOQuota deletes objects in FIFO order until sufficient objects
 // have been deleted so as to bring bucket usage within quota.
-func enforceFIFOQuotaBucket(ctx context.Context, objectAPI ObjectLayer, bucket string, bui BucketUsageInfo) {
+func enforceFIFOQuotaBucket(ctx context.Context, objectAPI ObjectLayer, bucket string, bui madmin.BucketUsageInfo) {
 	// Check if the current bucket has quota restrictions, if not skip it
 	cfg, err := globalBucketQuotaSys.Get(bucket)
 	if err != nil {